Superconducting materials, or superconductors, have an associated critical temperature (T.sub.c) that varies, depending upon the particular superconductor material. When a superconductor is at a temperature that is higher than the associated T.sub.c, the superconductor does not conduct electricity in an efficient manner. When the temperature of the superconductor is reduced to below the associated T.sub.c, it becomes an efficient conductor of electricity. Superconductors having a relatively low T.sub.c (near 20.degree. Kelvin) have been available for a considerable period of time. More recently, superconductors have been developed which have a relatively high T.sub.c (approximately 90.degree. Kelvin), which is above the temperature of liquid nitrogen (77.degree. Kelvin), thereby permitting such superconductors to be inexpensively maintained below their critical temperatures. Techniques are known for forming, upon suitable substrate material, a thin layer or film of High T.sub.c superconductor material.
However, a technique is required for inspecting the quality of large area superconducting films, particularly films comprised of high transition temperature materials, such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y, before large area devices can be produced with an acceptable yield. In particular, a tool is required to measure the T.sub.c, and the critical current density J.sub.c, of a superconducting film. The tool measurement resolution should be such that it can be insured that a device fabricated from the film will not be defective because of a defect in the original film.
It is thus one object of the invention to provide apparatus and method for determining at least one characteristic of a superconducting film material.
It is another object of the invention to provide apparatus and method for determining at least one characteristic of a superconducting film material, such as the critical current density and/or the critical temperature, at a localized region of the film.
It is a further object of the invention to provide apparatus and method for determining the presence and location of, and also the spatial extent of, a defect within a superconducting film material.